The U.S. Food and Drug Administration is stopping shipments of green onions from three Mexican companies at the border, as part of its investigation into an outbreak of hepatitis-A in several U.S. states.

Health officials are examining the shipments for traces of the disease which has sickened people in the U.S. states of Tennessee, Georgia, North Carolina and most recently, Pennsylvania. The Pennsylvania outbreak has been the most severe, infecting about 575 people and causing three deaths.

Previous shipments from the three Mexican suppliers have been linked to outbreaks of the liver disease in Tennessee and Georgia.

On Thursday, Food and Drug Administration officials said green onions are suspected of being the source of the hepatitis-A outbreak, but they do not yet have conclusive proof.